With the proliferation of mobile terminals, such as smart cell phones, RF transmitter design has progressed rapidly as designers try to minimize components and reduce size, battery consumption, and the like. Likewise, modulation schemes are continuously updated to reflect new approaches to maximize information transfers in limited bandwidths. Changes in standards or standards based on newly available spectra may also cause designers to approach modulating transmitters with different techniques.
If a polar modulation scheme is used, a phase modulated signal at the desired radio frequency is provided to the input of the power amplifier and an amplitude modulation component is used to vary the supply voltage provided to the power amplifier. As a result, the power amplifier may operate in saturation and efficiency is greatly improved. Unfortunately, the amplitude modulation component that controls the supply voltage provided to the power amplifier causes unwanted phase components to be created in the output of the power amplifier due to the non-linearities of the power amplifier. This is sometimes called Amplitude Modulation-to-Phase Modulation (AMPM) distortion, and it degrades the spectral purity of the system and an Error Vector Magnitude (EVM).
In order to compensate for the AMPM distortion, polar modulation systems may pre-distort the phase modulation component provided to the power amplifier such that the AMPM distortion at the output of the power amplifier is substantially reduced. As an example, see commonly owned and assigned U.S. Patent Application Publication No. 2003/0215025, entitled AM TO PM CORRECTION SYSTEM FOR POLAR MODULATOR, filed May 16, 2002, which is hereby incorporated by reference in its entirety. One issue with the AMPM pre-distortion, or compensation, is that the AMPM distortion may change with temperature, age, or other factors during real time operation. As such, circuitry providing the AMPM pre-distortion should be re-calibrated as needed when factors affecting AMPM distortion change. Therefore, there is a need to measure AMPM distortion during real time operation and use the measured AMPM distortion to correct for the AMPM distortion.